Service life of an aircraft is currently longer than in previous years and it is foreseeable and expected that service life will continue to increase in the future. The service life for many aircraft is longer and thus components of the aircraft are being utilized, in operation of the aircraft, for longer periods of time than originally intended and designed.
Due to the longer operating lives of the aircraft components, concerns have been expressed relating to fatigue life of the components. In particular, crack initiation, crack growth, and related topics have become of interest. Increased fatigue has caused an increasing number of cracks to develop and an increased amount of growth of existing cracks in aircraft components. Crack initiation and growth is also a concern due to potential inability to operate the aircraft, time and costs involved in repair of the cracks, and reoccurrence of crack growth after repair. Repair of cracks in both military and commercial aircraft is costly and generally crack repair is only a temporary solution.
Typically, crack growth is impeded or repaired using one of the following methods. Impedance of crack growth is facilitated by drilling a hole at each end of the crack, which is sometimes referred to as “stop drilling”. Stop drilling is only a temporary fix, typically the cracks over time return to growing, since area surrounding the crack is fatigued and the additional holes further weaken the component.
One method of repairing a crack includes use of a composite patch, which is applied through use of a structural adhesive over and directly to the crack and adhered to and forming a bond with the component of interest. The composite patch transfers load normally experienced on portions of the component near crack ends to areas surrounding the crack. The adhesive is typically an epoxy, but may be a form of glue, paste, or adhesive tape. The composite patch is preferred when higher strength is desired for a particular structural area.
Another method of repairing a crack includes application of a metallic patch over the crack. The metallic patch is fastened to the component of interest, also forming a bond with the component, via multiple fasteners such as rivets or bolts, which is labor intensive. The metallic material is preferred when the component of interest is utilized in an application that exhibits large temperature variances. In large temperature varying applications it is also preferred that the patch be of similar or same material as that of the component such that the component and the patch have similar expansion and contraction properties. The bond between the patch and the component withstands temperature changes better when the patch and the component are of similar material.
Although, the repairing methods are more durable and hold up for a longer period of time than the stop drilling method, they too are only temporary. Thus, none of the above-described methods fully repair or eliminate cracks and eventually the cracks return to growing.
It would therefore be desirable to provide a more robust crack repair technique, which provides a more permanent solution to crack initiation and growth.